In reality the watch is smaller than the Swiss Federal Republic's watch.

Princess Diana received "Lady Kalla" watch as a wedding gift. At the time it was the most expensive watch ever made by Vacheron.

The single-edition Lady Kalla was designed for Diana, Princess of Wales, and given to her as a wedding gift from a secret Arab sheik. Carved from 18-carat white gold, the watch was made of 108 emerald-cut, perfectly matched diamonds, with 26 more on the dial, adding up to 30 carats of diamonds with 3,172 facets. It cost $500,000. Source: the book "Time in Gold" by Viola and Brunner"

Perhaps Princess Diana wore it 1982/12/09 - LONDON 1° FILM

The Queen then passed the Swiss Federal Republic's watch down to Princess Diana in 1981, as a congratulatory gift before her wedding to the Prince of Wales.